Photocatalysts responsive to sunlight or indoor light to adsorb, decompose, and remove environmental contaminants, or exhibit a self-cleaning action to the dirt adhering to surfaces of objects have received attention these days, and have been studied energetically. Titanium oxide is one of the typical photocatalysts, and exhibits strong photocatalytic activity. Titanium oxide, however, has a large bandgap and has no absorbing properties to visible light that constitutes most of the sunlight. Titanium oxide exhibits photocatalytic activity by ultraviolet light, but does not exhibit activity by visible light. For this reason, titanium oxide cannot sufficiently utilize sunlight. Additionally, titanium oxide does not function in indoor situations having extremely weak ultraviolet light.
As measures against these problems, studies have been conducted to improve titanium oxide by doping titanium oxide with nitrogen or the like to enable absorption of visible light by titanium oxide. Further, studies have been conducted to search for novel oxide semiconductors other than titanium oxide that are responsive to visible light to exhibit activity as a photocatalyst. It is also known that visible light-responsive semiconductor compounds such as tungsten oxide have a smaller bandgap than that of titanium oxide and can absorb visible light, and such visible light-responsive semiconductor compounds work efficiently as a visible light-active photocatalyst (visible light-responsive photocatalyst) by adding a proper cocatalyst such as CuO, CuBi2O4, copper ion, platinum, and palladium or carrying the cocatalyst on the surfaces of the visible light-responsive semiconductor compounds (PTL 1, for example).
Unfortunately, many of these semiconductor compounds and cocatalysts are unstable substances under a severe condition such as alkaline or acidic conditions, and have limitation in the range of application. For example, tungsten oxide is easily dissolved under an alkaline condition, and cannot be used as it is in places such as sinks in which alkaline detergents are used. For this reason, a visible light-responsive photocatalyst stable under alkaline and acidic conditions has been desired for use in various applications for kitchens and bathrooms in houses.
To use such substances stably under the alkaline and acidic conditions, various studies have been conducted in the related art. For example, to form a photocatalyst layer having high alkali resistance on the surface of a base material, a method is reported in which a primary coating is formed on the base material using a resin containing a compound such as zirconium, titanium, and aluminum; an intermediate layer is further formed using a composition containing a compound of zirconium, titanium, or the like; on the intermediate layer, a photocatalyst layer is formed using a composition containing a photocatalyst particle and a zirconium compound as a binder; thereby, the photocatalyst layer is prevented from peeling off from the base material (PTL 2). Moreover, for improving the alkali resistance of an antibacterial photocatalytic coating material, a method in which polyorganosiloxane and an acrylic polymer are added to an aqueous coating material containing a photocatalyst to form a composite material, and a method in which a photocatalyst dry powder is contained in an acrylic silicon coating material are reported (PTLs 3 and 4).
These are methods in which a base material surface or a photocatalyst is coated with a stable substance to protect the base material surface or the photocatalyst from an alkali. For a photocatalytic reaction, however, holes and electrons generated by irradiation with light need to reach the outer surface. Additionally, the holes need to decompose a reaction substrate such as organic substances by oxidation, and the electrons need to be consumed by reduction of oxygen in the air. If the photocatalyst is protected with such a coating, the holes and the electron both have to pass through a newly coated protecting substance layer and react with the substrate and oxygen on the surface thereof. For this reason, charge separation is not promoted efficiently, and therefore the holes and the electrons are undesirably recombined and the reactivity of the surface of the coating substance is reduced, which result in problems such as undesirably reduced photocatalytic activity. Further, a complex process for coating is needed, which increases cost. Moreover, a desired coating structure according to the purpose of application cannot be easily obtained.